Dry developers used in electrophotography are generally classified into one-component developers using just a toner having a colorant dispersed in a binder resin and two-component developers comprising a toner and a carrier. In effecting duplication using such developers, the developer must be improved in such characteristics as fluidity, anti-caking, fixation, charge acceptance and cleanability in order to be compliant to the process. One common practice for enhancing these characteristics is to add to the toner inorganic microparticulates having a smaller particle size than the toner particles, for example, silica and titania microparticulates.
As the copying speed increases, the recent electrophotographic art places a greater demand for further improvements in fluidity, charging stability and uniformity, and cleanability. Also for better image quality, smaller particle size toners are utilized. However, the smaller particle size toners are poor in powder fluidity than the conventional toners of ordinary particle size and their charging characteristics are readily affected by additives such as external additives. Then a choice of inorganic microparticulates such as silica microparticulates to be added to the toner becomes more important.
Since commonly used silica microparticulates are very fine as demonstrated by a primary particle mean particle size of 10 to 20 nm, they have a strong tendency to agglomerate together and are poorly dispersible, failing to help the toner fully exert fluidity, anti-caking and cleaning characteristics. Silica microparticulates contain impurities which affect the charging characteristics of the toner. If the impurity content of inorganic microparticulates varies between different manufacturing lots, the toner varies in charge quantity, which can cause a significant variation in the density of images developed therewith.